Skjold Field, Danish North Sea: Early Evaluations of Oil Recovery Through Water Imbibition in a Fractured Reservoir

Abstract

Summary. This paper reviews the performance of a fractured chalk oil reservoir in its first 3 years of production and evaluates future development options. The relatively low degree of undersaturation of the oil required that evaluations of pressure-maintenance potential be made early in the field's life. To this end, many performance data have been collected, primarily through the use of observation wells. The performance data [pressures and free-water level (FWL) in fractures] performance data [pressures and free-water level (FWL) in fractures] are presented and their evaluation is described. In particular, the evaluation of the oil production performance through water imbibition is discussed. This evaluation has led to the initiation of pressure maintenance through water injection. Introduction The Skjold field is a naturally fractured chalk reservoir that lies in the Danish sector of the North Sea. The field was discovered in 1977 and was placed on production in Nov. 1982. The field is currently being depleted at a rate of some 22,000 BOPD [3500 m3/d oil] through a single crestal producing well. The oil originally in place (OOIP) is estimated to be about 600 × 10(6) STB [95 × 10(6) stock-tank m3]. Cumulative oil production to the end of 1985 was 12 × 10(6) STB [1.9 × 10(6) stock-tank m3] or 2% of the OOIP. Before field development, water imbibition was recognized as an uncertain but potentially important factor in any estimate of ultimate recovery. It was also recognized that because the oil was only 500 psi [3.4 MPa] undersaturated, an early assessment of the imbibition efficiency would be required to evaluate the potential of pressure maintenance through water injection. Two observation wells were therefore drilled in the field to collect reservoir performance data, one before the start of production and one in performance data, one before the start of production and one in 1985. It was found that important information could be obtained by monitoring the reservoir pressure and the movement of the fracture FWL in the observation wells. This monitoring has created a unique opportunity to observe the imbibition process at work in a naturally fractured field with bottomwater drive. In this paper, the Skjold reservoir is described, the reservoir performance data are presented, and evaluations of future recover performance data are presented, and evaluations of future recover potential are discussed. The interpretation of the data indicates potential are discussed. The interpretation of the data indicates the potential for increased recovery from the field. As a result, a small-scale waterflood program is being implemented to evaluate the potential acceleration of recovery through water injection. Geology The Skjold reservoir is contained in a chalk structure formed by a slightly elongated salt piercement dome. The reservoir is flanked on three sides by a series of ring faults. An extensive system of crestal faulting can be identified on seismic data (Fig. 1). The productive formation of the field consists of Lower Tertiary (Danian) and Upper Cretaceous (Maastrichtian/Campanian) chalk, which varies in thickness from 250 to 700 ft [76 to 210 m], with the thinnest chalk on the crest. The total oil column height is about 1,300 ft [400 m]. Matrix rock porosity lies in the range of 15 to 30%, and the air permeability of nonfractured reservoir rock is usually less than 1 md. Small-scale faults and numerous microfractures can be identified on core material. These are believed to act in conjunction with the larger-scale faulting in a fracture system that extends throughout the reservoir. This fracture system provides a network of highly permeable fluid conduits that result in the high well productivities permeable fluid conduits that result in the high well productivities and excellent drainage characteristics of the field. A schematic cross section of the field is shown in Fig. 2, which shows the connection of the crestal region to the chalk on the flanks of the salt dome. This flank chalk is water-bearing and is the apparent source of aquifer pressure support. A volumetric calculation of OOIP yields an estimate of approximately 600 × 10(6) STB [95 × 10(6) stock-tank m3]. This is in good agreement with material-balance estimates of OOIP. Individual Well Behavior and Testing Four wells have been drilled into the Skjold reservoir, one (abandoned) discovery well, one producer, and two observation wells. The discovery well and the producing well were both drilled into the crestal portion of the structure, while the two observation wells were drilled into the thicker chalk on the flanks. The crestal development well currently produces 22,000 B/D [3500 m3/d] of 30 degrees API [0.88-g/cm3] oil at a solution GOR of 490 scf/STB [89 std m3/stock-tank m3]. The bottomhole pressure (BHP) drawdown at this rate is only about 60 psi [410 kPa]. This high productivity is clearly the result of an open fracture system permeating the tight (1-md) matrix rock. On the basis of this permeating the tight (1-md) matrix rock. On the basis of this productivity, the effective permeability to flow is estimated to productivity, the effective permeability to flow is estimated to be on the order of 1,000 md. Conventional transient buildup analysis is complicated by the nearly instantaneous buildup to static pressure when the well is shut in. Similar buildup response was observed in the crestal discovery well. The observation wells drilled into the thicker flank chalks show significantly lower productivity, but fracture enhancement is still evident. As an example, a semilog plot of buildup data from an observation well test is shown in Fig. 3. The data exhibit the characteristic S-shape that is considered typical in a dual-porosity reservoir, as described by Warren and Root. Using the analysis methods outlined by Bourdet and Gringarten, an effective permeability of 45 md, skin of -2.5, lambda of 10(-5), and w of permeability of 45 md, skin of -2.5, lambda of 10(-5), and w of 0.6 can be calculated from this test. Interference testing between the wells shows good communication and very high permeability between the crestal and flank wells (on the order of 1,000 md), despite the relatively low productivity of the flank wells. This is attributed to high-permeability flow channels, possibly along faults, which were not directly intersected by the possibly along faults, which were not directly intersected by the flank wells. SPERE p. 17